Every day, the water surges southward — the billion gallons rushing from serene upstate lakes and reservoirs down, down, down to sustain New York City. Without it, the city would go dry.

Which is what is so disturbing about allegations this week that Mayor Bill de Blasio’s administration played fast and loose with the funding for a key fail-safe of the city’s water-supply system serving Brooklyn and Queens.

Two tunnels carry the vast majority of New York City’s water from the Catskills. Built in the first half of the 20th century, the tunnels desperately need repair. To turn them off for renovation, the city began building a third tunnel in 1970 as an extra conduit. Completing this new tunnel, an essential infrastructure project, became a major priority for the Bloomberg administration, given that the city would have no way to get enough water to vast swaths of its people should one of the original tunnels fail.

Last year, the commissioner of the city’s Department of Environmental Protection indicated that funding for the construction of the last phase of the third tunnel, which is nearly complete, had been removed from the city’s preliminary budget and shifted to another priority. A New York Times report suggests the shift was precipitated by an attempt to keep water-bill rates down.

The mayor’s office denies the accusation, noting that the small amount of money in question — some $300 million for this construction — would have little effect on rates, the equation for which is partially dependent on capital needs. The DEP capital budget runs into the billions. In an effort to clarify the situation, de Blasio has said this is all a misunderstanding.

The mayor said the money was put on hold for a “budgetary reason” that had not held up work, and that he is reinstating the money anyway and announcing that the project will be sped up a year.

The mayor’s about-face is a welcome acknowledgment of how precious the city’s water system is, and a reminder that it is in urgent need of attention.

Not having an adequate backup plan to keep the spigots running, in all circumstances, is not an option.